If you suck your thumb will your breath stink?

If you engage in proper oral hygiene and don't have any underlying gum disease, sucking your thumb will not necessarily result in halitosis, the technical name for bad breath.

Usually there are two reasons for halitosis: odor from the stomach, usually based on some form of indigestion, or odor from the tongue (or from food lodged between the teeth) caused by the chemical results of certain kinds of bacteria.